1. Field of the Invention
The invention is generally related to the area of optical communications. In particular, the invention is related to a motor driven variable optical attenuator with IR sensor closed-loop control.
2. The Background of Related Art
The future communication networks demand ever increasing bandwidths and flexibility to different communication protocols. DWDM (Dense Wavelength Division Multiplexing) is one of the key technologies for such optical fiber communication networks. DWDM employs multiple wavelengths or channels in a single fiber to transmit in parallel different communication protocols and bit rates. Transmitting several channels in a single optical fiber at different wavelengths can multi-fold expand the transmission capacity of the existing optical transmission systems, and facilitating many functions in optical networks.
In general, the channel signals come from different sources and may have transmitted over different mediums, resulting in different power levels. Without equalizing the power levels of the channel signals that are to be combined or multiplexed, some channels in a multiplexed signal may be distorted as a result of various stages of processing the multiplexed signal. On the other hand, many optical devices or systems would not function optimally when incoming signals are beyond a predetermined signal level range. In fact, the power of the incoming signals shall not be too low, neither too high. To ensure that all optical devices or systems receive proper levels of optical signals, attenuation devices are frequently used to adjust the optical signals before they reach an optical device.
Many existing optical attenuation devices are open loop controlled due to lack of internal accuracy feedback signal. An electrical tuning variable optical attenuator (EVOA) is capable of quickly controlling the optical signal power. However, many of EVOA developed so far are either based on MEMS, or by moving a ND filter driven by a stepping motor with gear reduction mechanics and a potentiometer for positions, or waveguide based VOAs. While the ND filter based approach suffers from a slow adjustment speed and a high cost of components such as the ND filter and supporting optics and mechanics, the MEMS or other non-ND-filter approaches typically are too sensitive to have a fine attenuation resolution, and good device assembly repeatability. Waveguide VOAs are suitable for high channel-count integration, but have the issues of high polarization dependent loss, sensitive to ambient temperature. Their use in an open loop control often results in these undesired issues, such as high temperature-dependent loss, performance and reliability issues, rendering the shortening of their service life cycle.
To solve most of these issues, a closed-loop control has been applied by adding an EVOA and a tap optical filter or coupler together with a photo-detector (PD) as a sensor, as shown in FIG. 1A and FIG. 1B, respectively. The combination of tap and PD generates a small photocurrent proportional to the output intensity from the EVOA. Such a current serves as a feedback signal to further adjust the EVOA so as to meet the attenuation requirement. Such a tap-PD based EVOA design is often adopted in many optically devices or systems. However, the external feed-back mechanism makes an overall system higher in cost and introduces higher insertion loss and often makes the final system bulky.